The present invention relates to a method of determining the actual position of a probe head which is mounted to a coordinate-measuring machine via an articulating head (Dreh-Schwenk-Gelenk) having two orthogonally related axes of articulation.
Such articulating heads serve on coordinate-measuring machines primarily for angularly orienting the probe head used for coordinate-measurement of workpieces, in such manner that the probe head can be optimally used for the desired measurement task, for example, to permit successive entry of a probe into a plurality of holes of different orientation on the workpiece.
Known articulating heads can be divided into two classes: the first class comprises articulating heads of the so-called switching type, in which the probe head (mounted to the articulating head) can be set, via detents, in a limited number of discrete angular positions which differ from each other, for example by about 7.5.degree.. The other class comprises articulation heads of the type which can be adjusted continuously to any desired angle about either or both of their articulation axes and which contain encoders in both axes, for supplying the computer of the coordinate-measuring machine with measurement values corresponding to currently set angular values.
A switching-type articulating head is illustratively described in U.S. Pat. No. 4,313,263, and a measuring-type articulating head is described in German OS 37 40 070, corresponding to U.S. Pat. No. 4,888,877.
A probe head that is positioned via a two-axis articulating head may be mounted to the articulating head either directly or via an extension piece. In either event, the position of the probe head or, rather, the position of the work-contacting ball at the tip end of a probe pin mounted to the probe head, must be known very precisely in all angular positions, if very precise coordinate measurements are to effected. But such parts can sag within a range of several times 10 .mu.m, due to their own weight. As long as the angular position of the probe and of its probe extension remains unchanged, the amount of sag is constant and therefore does not affect the coordinate measurement on the workpiece, which, after all, is always a relative measurement. But it is unrealistic to expect such a condition during the operation of a probe head mounted to an articulating head, because the whole point of an articulating head is to bring the probe into different angular positions; the amount of the sag will therefore change, depending, for example, upon whether the probe head is oriented horizontally or vertically. In the latter case, the sag is practically zero. In the former case, on the other hand, the sag may assume values in the order of magnitude of up to 180 .mu.m.
Until now, and in particular for articulating heads of the switching type, it has been customary, before making a measurement with a newly inserted probe head, to follow a calibration procedure in which the existing probe coordinate, i.e., the position of the probe-pin ball mounted to the probe head, is determined and digitally stored for all angular positions required in the course of the measurement program to be followed. In this way, the influence of gravity on the position of the probe, i.e., the effect of sag due solely to the weight of the probe head and of its extension is automatically determined. Such a method of calibration is, however, very time-consuming and cumbersome, since the position of the probe head must be redetermined in each angular position of the articulating head, by making repeated contact with a calibration ball, and performing associated operations involved in setting up the ball for such calibration.
It is known from U.S. Pat. No. 4,663,852 that, for a work-contacting probe carried by the measurement arm of a coordinate-measuring machine, (1) probe-pin deviations attributable to a bending of the measurement arm, with respect to a reference line or plane, may be detected by attached inclinometer instrumentation, and (2) that measurement values from the inclinometer instrumentation can be used to correct current instantaneous probe head or probe-pin ball measurement values. This method, however, cannot be employed when the angular orientation of the part in question is to be intentionally variable, as for example in the case when using a two-axis articulating head. Furthermore, it is not readily possible or practical to install one or more sensors in a probe head in order to measure instantaneous inclination.
EP 0,147,529 (corresponding to U.S. Pat. No. 4,611,156) teaches that one may correct for such bending deflections of the probe pin of a probe head as may occur in measurement made in a scanning procedure, wherein the bending deflections are a function of scanning speed and/or of differences of materials, and these factors are taken into account. But this known technique is also not adapted to correcting for such deviations in position of the probe-pin ball as are attributable to sag that is dependent on instantaneous angular orientation and position of the probe head.